The smoke fire detection technique based on the light scattering principle of an aerosol has been widely applied since 1970s when the technique was first used. However, the surface area and particle size of an aerosol cannot be sensed or fire smoke cannot be distinguished from dust and steam in the prior art, and therefore false alarm becomes the biggest factor affecting detection effectiveness.
In general, the particle size of a fire aerosol generated from material burning is smaller than 1 μm, and the particle size of a non-fire aerosol such as steam and dust is larger than 1 μm. For the same mass concentration, a small-size aerosol is more in particle number and large in surface area, a large-size aerosol is less in particle number and small in surface area, and therefore a fire aerosol and a non-fire aerosol can be distinguished more effectively based on the surface area concentration of the aerosol and other characteristic parameters such as mass (volume) concentration and Sauter diameter of the aerosol all together.
Chinese patents with the patent No. 200410031104.5, the patent No. 200980138873.6 and the patent No. 201180039383.8 all disclose methods of distinguishing aerosol particles with diameter larger than 1 μm and smaller than 1 μm by using scattered light signals with two different wavelengths, so as to reduce the false alarm rate of a fire smoke alarm. However, the specific particle size value and surface area concentration cannot be sensed by using these methods. A Chinese patent application with the application No. 201410748629.4 discloses a method of sensing the median particle size of an aerosol using scattered light signals with two different wavelengths, but the method cannot sense the surface area concentration of an aerosol. The document “Greenberg, P. S. and Fischer, D. G., Advanced Particulate Sensors for Spacecraft Early Warning Fire Detection, Paper No. AIAA2010-6243, 40th International Conference on Environmental Systems, Barcelona, Spain, Jul. 11-15, 2010” provides a method of measuring the surface area concentration and the mass concentration of an aerosol using a specific optical structure working at the same wavelength and different scattering angles. However, according to the aerosol Mie scattering principle, balanced response of large and small particles can hardly be achieved with the same wavelength, and the measurement error of the method is large.
To overcome the defects in the prior art, a Chinese patent application with the application No. 201410748629.4 provides a method of sensing a particle size of an aerosol using dual-wavelength light scattered signals capable of identifying different types of fires and steam and dust interference according to a median particle size value and giving alarms with corresponding alarm signals. The method comprises the steps of calculating the ratio R of scattered light power of blue light to scattered light power of infrared light after receiving corresponding scattered signals of the aerosol, expressed by the scattered light power PBL of blue light and the scattered light power PIR of infrared light; determining a median particle size dmed according to the relationship between the ratio R of scattered light power of blue light to scattered light power of infrared light and the median particle size dmed of the aerosol; and comparing the scattered light power PBL of blue light and the scattered light power PIR of infrared light with corresponding set thresholds PBLth and PIRth and emitting corresponding interference prompt signals or corresponding fire alarm signals. Though the method can be used for judging and emitting fire type alarm signals so that targeted and rational measures can be taken and a non-fire aerosol false alarm can be avoided to a certain extent, the median particle size of the aerosol cannot be obtained directly due to the fact that the ratio R has no corresponding physical meaning; and it is required that an experiment be conducted on R in advance so as to obtain a particle size spectrum curve covering all particle sizes from small to large and store the particle size spectrum curve, and only through comparison and search can the particle size be obtained, which is both inconvenient and inaccurate. Specifically, it can be learned from a curve of a non-linear relationship between R and the median particle size of the aerosol, for particle sizes smaller than 200 nm and larger than 1,000 nm, the ratio is in a non-linear area, making it difficult to obtain an accurate result, and for particle sizes larger than 1,500 nm, distinguishing fails due to the fact that the ratio R changes too slightly. Besides, as the surface area concentration of the aerosol cannot be obtained with the method, the small-particle size fire aerosol with large surface area concentration but small mass (volume) concentration cannot be effectively sensed. Furthermore, the characteristic parameters of the aerosol are represented by amount concentration, surface area concentration, mass (volume) concentration and particle size distribution, and the larger the number of sensed characteristic parameters is, the more accurate the judgment tends to be.